Flat beam cathode-ray tube and circuit



April 13, 1948 L. F. BROADWAY A 2,439,504

FLAT BEAM CATHODE RAY TUBE AND CIRCUIT Filed Marh e, 1945 H VVENTOR.LeazzafaFm/zczf Ervan/way l ATTORNEY Patented Apr. 13, 1948 FLAT BEAMCATH AND CIR ODE-RAY TUBE CUIT Leonard Francis Broadway, Ickenham,England,

assigner to Electric & Musical Industries Limited, Hayes, England, acompany of Great Britain Application March 6, 1945, Serial No. 581,281

' In Great Britain March 9, 1944 (Cl. o-158) 7 Claims.

This invention relates to cathode ray tubes and circuit arrangementsembodying such tubes.

For some purposes cathode ray tubes are required in which thesensitivity of deiiection of the beam across the screen of the tube inone direction is required to be much greater than the sensitivity ofdeflection in another direction perpendicular to the first. For example,in cathode ray tubes employed in radar equipment a high -sensitivity ofdeflection is required in the signal direction and also in cathode raytubes employed for television purposes the sensitivity of deection isrequired to be much greater in the direction of the line scan than inthe direction of the frame scan.

Usually the cathode ray beam is scanned across the screen of the tubeeither by. electrostatic deflecting means cr by electromagneticdeflecting means, each deflecting means comprising either a pair ofplates or a pair of coils which may be associated with magneticpolepieces provided within the envelope of the tube. It is known that inorder to obtain optimum sensitivity of deection the deflecting plates orthe pole pieces of the coils should be placed as close as possible tothe electron beam. Since the beam, which is lusually of circular form incross-section, is brought' to a focus at the screen and since it isnecessary to dispose the deecting means at some distance from the screenin order that the focussed beam can scan an area of a desired size, thedeflecting means must be disposed at a position where the diameter ofthe beam is fairly large which results in the sensitivity of deflectionbeing relatively small. If it is desired to reduce the diameter of thebeam at the position Where theA deflecting means are disposed, then itis necessary to step down the beam diameter which is an undesirableexpedient owing to the consequent, reduction in the beam current.

It is the object therefore of the present invention to provide animproved cathode ray tube and circuit arrangement embodying such tube inwhich the sensitivity of deiiection can be increased in one directionwithout necessitating a decrease in the available beam current.

According to one feature of the invention there is provided a circuitarrangement embodying a cathode ray tube in which the electron beam ofthe tube is of elongated form in cross-section, said tube having meansfor causing said elongated beam to converge to form a small spot. on thescreen of the tube and in which deflecting means are provided fordeecting said spot relatively to said screen in one direction and de-Cil fleeting means are provided for deecting said spot relatively tosaid screen in a co-ordinate direction, deflection inone directionoccurring at a more rapid rate than the deection in the other direction,wherein the deilecting means for deiecting the spot at said more rapidrate are disposed at a position Where the beam is of elongated form` andembrace the longer sides of the beam cross-section thus permitting saiddeecting means t0 be placed closer together than would otherwise bepossible, whereby the sensitivity of deflection at said more rapid rateis increased,

According to another feature of the invention there is provided acathode ray tube having means for generating an electron beam ofelongated form in cross-section and having means for causing saidelectron beam to converge to form a small spot on the screen of the tubeand in which deiiecting means are provided for deflecting said spotrelatively to said screen in one direction and deiiecting means areprovided for deecting said spot relatively to said screen in aco-ordinate direction, one of said deflecting means being capable ofdeflecting the spot at a more rapid rate compared with the rate ofclelection produced by said other deflecting means, wherein thedeflecting means capable of deecting the spot at said more rapid rateare disposed at a position where the beam is of elongated form andembrace the longer sides of the beam crosssection thus permitting saiddeecting means to be placed closer together than Would otherwise bepossible, whereby the sensitivity of deection at saidmore rapid rate isincreased.

Usually, the deflecting means are disposed at separated points along thepath of said Abeam and the deecting means which deect the beam at themore rapid rate are disposed in a position Where the beam is moreelongated compared with the elongation of said beam at the point wherethe other deecting meansI are disposed.

Witha cathode ray tube employed in accordance with the invention it ispossible to arrange that the beam is of a substantially rectangular formin crosssection, the depth of the crosssection being, for example, ofthe order of 0.005, the width of the beam being of any suitable size inorder to obtain a desired beam current. Where the cathode ray tubeemploys electrostatic deiiecting plates the distance apart from saidplates at the point Where they approach closest to the beam may be onlyslightly larger than the depth of the beam thus providing a substantialimprovement in the sensitivity of deflection.

Where electromagnetic deflecting means are employed the pole pieces -ofthe deflecting coils need only be spaced apart by a distance slightlylarger than the depth of the beam although in this case it will b eappreciated that the direction in which increased sensitivity ofdeection occurs will be at right ang-les to that obtained withelectrostatic 4 As shown in the drawing-the referencemlmeral l indicatesthe envelope of a cathode Iray tube, having a screen S. VThe referencenumeral 2 indicates a cathode of elongated form`mounted within acathodeshield 3 of rectangular form, the shield' 'having a rectangularslot'll, `as shown, for defining vthe size of the beam emitted from thecathode 2. The shield L3 in, for example, a television reproducing `tubemay serve as a modulating electrode to which modulated potentials can beapplied for varying the intensity of the beam. The beam emanating4through the slot 4 is thus of elongated rectangularform and the depth ofthe slot may be Say 0.005" whilst its width willbe of a suitable size topermit the passage of a beam of a desired current. `Adjacent to theshield 3 (is a tubular electrode 5 of rectangular form in cross-section,the end of the electrode adjacent the shield `3 being closed by a plateprovided with a rectangular slot 6 therein. The electredev may bemaintained at apositive potential when operating the tube and lits slotYt3, in conjunction with Vthe -slot vl and the potential applied inoperation tothe shield 3, lwhich will be negative with respectto ytheelectrode '5, serves to produce lan electrostatic lens which maintainsthe beam emanating through the slot sensibly parallel or slightlyconvergent, with Vrespect to the plane containing the major axis of theslot il; thus the operation of the electrostatic lens should be suchthat the velectron beam does Anot increase in depth Y-beyond the depthof the' slot 4. It may in some casesbe desirable to insert a furtherelectrode or electrodes suitably slotted yand maintained at suitablepotentials Vbetween Vthe electrodes 3 and or'thesurface ofthe cathode besuitably curved in its shorter dimension order to ensure this result.The electron Ybeam indicated bythe dotted lines l' passing through theelectrode 5 is'then acted upon'by a further electrostatic lens in orderto causeV the beam to converge so as --to form a small spot of a desiredsize on thescreenrS. This converging action may be accomplished vbyemploying an electrostatic lens of one of the .types described .in thespecification of British patent application No. 4628/42. Such a lens maycomprise afurther tubular electrode 8 of rectangular form Vincross-section which cfs-operates with the end of the electrode 5reinotefrom the cathode, the adjacent ends of the electrodes 5 .andbeing suitably shaped and provided with slots in the ,manner describedin the aforesaid specification so as lto afford a convergeht beam, theaction of the lens formed by the electrodes5 .and 8 being such thatthedepth of the beam is substantially unchanged whilst its width'convergesto form a spot of the desired size. v In operation the electrode 8 willusually be maintained at, a hgherpotential than the electrode 5 and theelectrode 8 may be provided with a slot 9 in order to define the size ofthe beam emanating from the electrode 8.

Deecting means are provided for deflecting the beam relatively to thescreen of the tube in Vonedirection and deflecting means are providedVfor deflecting said beam relatively to said screen in a co-ordinatedirection. As shown in the drawing, the electron beam is arranged to bedellected ein -a directionfperpendicular to the plane containing themajor axis of the beam cross-section Y, by a pair of plates I0 to whichsuitable poten- -tials are applied -for setting up an electrostaticdeeetinvgjeld. 4The pair of plates I0, it will be observed, embrace thelonger sides of the beam cross-section and are disposed in a positionwhere .fthe beam'is stillof elongated form. The deflecting means ll fordeflecting the beam at right.. angles -to the deflection imparted by theplates l0, are spaced from the plates l0 along the path of the beam andare closer to the screen .S of the tube than said plates l0 so that thebeam at the point where the other deflecting means ll are disposed willbe less elongated compared with the elongation of the beam at theApOilltwhere the plates `lll are disposed. AIn oneof the directions of deectionthe beam is required ,to'be deflected at a more rapid rate compared,withthe rate of deflection in the other direction. Yinthe case of acathode ray tube employed in Ytelevision apparatus deflection of thebeam at -the linescanning frequency is far more rapdthan'the deflectionbeamat the frame-scanning frequency. With the tube shown in the drawingthe plates l0 will serve to deflect the beam at the more rapid ratewhilst the other deecting means `l l, (not shown), will serve to deflectthe beam at the slower rate. By arranging the plates lin the positionshown the sensitivity of deflection at the more rapid rate is vincreasedsince the distance apart of the edges of the plates 1-0 where they areclosest to the beam need only b eslightly larger than the depth of theVbeam, i. e.; slightly larger than 0.005". In-this manner thesensitivity of dellection can thus be made very'high whilst at the sametime the plates l0 can'be disposed at a position jto permit an area of adesired Asize ,to be scanned. The deflection of the'beam Vin the otherdirection maybe accomplished `by the'further pair of defleoting plates`II orbyelectromagnetic deflecting means. If desired, of course, theplates I'U ,may be replaced by the polepieces of electromagneticdeflecting means the pole pieces extending within the envelope l andbe.-.ing disposed close to the beam. vWhere .the plates l04 are replaced 'bypole pieces it will of course be appreciated. that the deflection will[be in a direction mutually perpendicular to .thedirection of deectionproduced by `the ,plates lll.

It is possible that with the construction described employing a beam o felongated cross-section, a higher modulationslopeand a lesscurvedmodulation characteristic may .be obtained compared with a tubeemployinga vcircular,cathode. Furthermore, less .de-focussing may occurwith the modulation potentials ,and with the .potentials applied to thedelecting plates.

I claim:

1. A circuit arrangement embodying a cathn ode ray tube having ,means.for produclnga thin at electron beam rectangular in cross-section.means ,for causingsaid flatbeam to converge in Width to forma smallspot,Qn thescreen of the tube deilecting means fordeflecting said spotrelatively to s aid screen in one Idirection ,and deflecting means fordeflecting said spot relatively to said screen in a co-ordinatedirection, de dcction in said one direction occurring at a more rapidrate than the deection in the other direction, wherein the defiectingmeans for deiiecting the spot at said more rapid rate are disposed a,J aposition where the beam is rectangular in cross-section to embrace thelonger sides of the beam cross-section and placed closed togetlier thanwould otherwise be possible, whereby the sensitivity of deflection atsaid more rapid rate is increased.

2. A circuit arrangement according to claim 1, wherein said deflectingmeans are disposed at separated points along the path of said beam andthe deecting means which deect the beam at the more rapid rate aredisposed in a position where the at beam is wider than at the pointwhere the other deflecting means are disposed.

3. Television apparatus incorporating a circuit arrangement according toclaim 1, in which said deecting means serve respectively to deect thebeam of the tube at the line-scanning frequency and at theframe-scanning frequency, said deflecting means which embrace the longersides of the beam cross-section serving to deflect the beam at theline-scanning frequency.

4. A cathode ray tube having means for generating an electron beamrectangular in crosssection and having means for causing said electronbeam to converge to form a small spot on the screen of the tubedeflecting means for deiiecting said spot relatively to said screen inone direction and deflecting means for deecting said spot relatively tosaid screen in a co-ordinate direction, one of said deecting means beingdisposed at a position where the beam is rectangular in cross-section toembrace the longer sides of the beam cross-section, said deecting meanscomprising parallel members spaced a distance only slightly greater thanthe depth of the cross-section of said beam between said memlbers,whereby the sensitivity of deection at said more rapid rate isincreased.

5. A cathode ray tube according to claim 4, wherein said delecting meansare disposed at separated points along the path of said beam, one of thedeecting means being disposed at a point where the width of the beam isgreater than at the succeeding point along the path of the beam wherethe other deecting means are disposed.

6. A cathode ray tube comprising an evacuated envelope enclosing meansfor producing a thin at beam of electrons, a fluorescent screen in thepath of said beam, beam focusing means eifective only on the width ofsaid beam to focus said beam to a spot on said screen, deflecting meansfor producing a deiiecting eld perpendicular to the sides of said datbeam at a region near the said focusing means Where said beam is wide, adeecting means for producing a deecting field parallel'to the sides ofsaid beam at a region more remote from said focusing means and where theWidth of said beam is less than at the first deiiecting eld.

'7. A cathode ray tube having a thermionic cathode, a beam formingelectrode for forming the electron discharge from said cathode into athin fiat beam rectangular in cross-section, a tubular acceleratingelectrode for said beam, a focusing electrode adjacent the beam exit endof said accelerating electrode for forming with said acceleratingelectrode an electrostatic lens effective to converge said beam only inthe direction of its width to focus said beam, a pair of deecting platespositioned near said focusing electrode and disposed on opposite sidesof the normal path of said beam to be parallel to the sides of said flatbeam and spaced a, distance only slightly greater than the depth of thecrosssection of the beam between said plates to deflect said beam in adirection perpendicular to the sides of said beam, and a second pair ofdeflecting plates more remote from said focusing electrode andpositioned to deflect said beam in a direction parallel to the sides ofsaid flat beam.

LEONARD FRANCIS BROADWAY.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,036,532 Knoll et al. Apr. 7,1936 2,053,537 Schlesinger Sept. 8, 1936 2,081,942 Lubcke June 1, 19372,152,363 Ruska Mar. 28, 1939 2.183.309 Flechsig et al. Dec. 12, 1939

